1. Field of the Invention
The present invention relates to an apparatus for measurement of an optical pulse shape intended to measure the temporal waveform of an ultrashort single optical pulse.
2. Description of the Prior Art
In the prior art, the conventional method of measuring the waveform of an ultrashort optical pulse converts an incident optical pulse into an electrical signal by use of a fast-photo diode, and measures the signal by a wide-bandwidth oscilloscope. However, the pulsewidth of the optical pulse that is able to be measured by a fast-photo diode and wide- bandwidth oscilloscope is limited in about 50 ps. Therefore, a streak camera is used for measuring the waveform of an optical pulse having a pulsewidth of a few ps, however, it can only measure in the range from near ultraviolet to near infrared and has a high-price. Thus, it is required to develop a measurement apparatus that is able to measure the waveform of a short pulse having narrower than a few ps pulsewidth or for a spectrum in which a streak camera can not measure. On the other hand, dislike the direct measurement method of the waveform, a pulsewidth, which is one of the important informations of an optical pulse, can be determined by measuring the second order auto-correlation using a nonlinear optical effect. In this method, the second harmonic lightwave and two photons absorption is mainly used to produce a nonlinear effect.
FIG. 1 is a view illustrating the structure of a prior-art apparatus for measurement of the pulsewidth of an optical pulse using the second harmonic lightwave.
A nonlinear optical effect causes interactions between numerous incident lights and produces a new light with a frequency of the sum or difference of the frequencies of the incident lights. In order words, it produces a light with different wavelength (Here, the frequency and the wavelength are inversely proportional to each other.). The process that two lights, each has the same frequency of .omega., interact each other in a nonlinear optical medium and produce a light with a frequency of 2 .omega. is called the second harmonic lightwave generation. The procedures to measure the pulsewidth using the second harmonic lightwave is a follows:
At first, one divides the optical pulse 1 to be measured into two optical pulses using a 50:50 optical beam splitter 2 and makes the two divided lights proceed in different routes respectively. At this stage, a time delayer 3, which is able to control the time delay between the two lights, is located in one route and a reflection mirror 4 is located in the other route. The two lights is combined by a 50:50 optical coupler 2, then the combined light passes through a lens 5 and incidents to a nonlinear optical crystal 6. The second harmonic lightwave is generated thereon according to the intensity of the light. Since the second harmonic lightwave comes out with the light having a frequency of .omega., one should eliminate the light having a frequency of .omega. by a filter 7 and thereafter, measures the second harmonic lightwave by a photo diode 18 or a photo multiplying tube (PMT).
In the procedures, the intensity of the second harmonic lightwave is determined by the overlapped amount of two optical pulses when they are coupled. In other words, in case that the time delay between the two lights is so large that they can not be overlapped when they are coupled, the intensity of the second harmonic lightwave becomes very small, and in case that the time delay is zero, the intensity of the second harmonic lightwave gets its maximum value. Therefore, if one operates the time delayer 3 with a fixed velocity and measures the intensity of the second harmonic lightwave generated, one can find the amount of overlapped pulses and measure the pulsewidth thereby. This method, however, has a drawback that one should assume the waveform of a pulse to find an accurate pulsewidth.
FIG. 2 is a view illustrating the structure of a prior-art apparatus for measurement of the pulsewidth of an optical pulse using two photons absorption effect.
A light-absorbing material has an energy band gap corresponding to the energy of the incident light. In the energy band gap of the material is bigger than the energy of the incident light, the light is not absorbed but transmits. However, the intensity of the incident light gets high, an absorption still happens in this case. This phenomenon is one of nonlinear phenomena of the medium, and it happens in a medium of which the energy band gap is twice as much as the energy of the incident light. This is called two photons absorption effect. The two photons absorption effect increases proportional to the intensity of the incident light. The apparatus for measurement of the pulsewidth of an optical pulse using the two photons absorption effect is similar to that of the method using the second harmonic lightwave as described in FIG. 1. The procedures to measure the pulsewidth using the two photons absorption effect is as follows:
One divides the optical pulse 11 to be measured into two optical pulses using a 50:50 optical beam splitter and makes the two divided lights proceed in different routes respectively. At this stage, a time delayer 13, which is able to control the time delay between the two lights, is located in one route and a reflection mirror 14 is located in the other route. The two lights is combined by a 50:50 optical coupler 12. The combined light passes through a lens 15 and is measured by a photo diode 18.
The difference between the two apparatuses described in FIGS. 1 and 2 is that the latter does not use a nonlinear medium but make use of the two photons absorption effect of a photo diode used for measurement. This method has advantages that it has simpler structure than the method of using the second harmonic lightwave and the fabricating price is low. However, it has a drawback that the sensitivity is lower than that of the method of using the second harmonic lightwave. And with this method, one should also assume the waveform of a pulse to find an accurate pulsewidth.